Simon explains the Grimoire Synthesis:
"You can combine three separate Grimoire Stones into a single one. This is called Grimoire Synthesis."
"The first Grimoire is the base, which determines the number of skill slots in the final result."
"You can choose skills from the first and second Grimoires you use to place in the resulting Grimoire."
"The third Grimoire doesn't carry over any skills, but you can transfer the equipment bonus into the result.

But even that explaination can be confusing (even to an old head like me). So here are some visual examples which should help.
(Ive noticed that the Equipment Bonus is based on which Character created the stone: Simon=Staff, Raquna=Sheild, Arthur=none, Frederica=Gun, Unknown=random)

I hope these examples help make sense of it all.

Rule of thumb: your BASE stone should be your BEST stone that you want to keep. It should contain the highest number of slots you want.
For the Synthesized stone: your most powerful skills should be in this stone.
For the Catalyst stone: your stone with the most number of Equipment Bonuses should be used.

But how do I know how many slots a stone will have? I havent figurd that out yet. (I thought Copper=1, Dark=2, but I found a Dark with 1 slot and Copper with 3)


Now for a multi-synthesis process: